I. Field of the Invention
This invention relates to a process for improving the color and oxidation stability of feedstreams which contain multi-ring aromatic and hydroaromatic compounds. In particular, it relates to a process for improving the color and oxidative stability of hydrocarbon feedstreams which contain multi-ring aromatics and their partially or fully hydrogenated derivatives, especially hydrocracked feedstocks of such character useful as lubes basestocks.
II. Background and Prior Art
In hydrogenation reactions, especially those conducted at high severities, or at temperatures and pressures less than optimum for some period of a total operating cycle, there is often a decline in product quality manifested by higher color, or propensity of the product to become discolored, or further discolored, on exposure to light or oxygen, or both. Hydrogenation, or hydroconversion reactions, e.g., hydrocracking reactions, thus produces liquid products which are unstable in the presence of light and oxygen (air). This instability manifests itself in the form of color degradation to produce darker liquids, or solid sediments, or both; the sedimentation usually appearing as a dispersed phase which clouds and discolors the liquid products as dark colored precipitates. Degradation of products in this manner occurs sometimes immediately, and ofttimes after the products have been stored for some period; often very short periods. Such degradation often causes performance problems in the ultimate end products; or, may simply limit customer acceptance of the products. Color and oxidative stability is especially important in the production of lubes, for which reason lubestocks and finished products produced therefrom must meet a number of rigid stability tests to assure both product performances and customer acceptance. For example, hydrocracked feedstocks, such as recycle streams obtained from high pressure hydrocrackers, must meet these rigid stability tests. Such feedstocks can be considered useful lubes basestocks, if they can meet "daylight stability" test requirements, an accelerated aging test werein the sample is exposed to air and light at elevated temperature.
The daylight stability test indicates the propensity for a lubestock to darken in color, or throw a sediment, or both, when exposed to light and oxygen under specific conditions. It is believed that this type of instability is caused by the presence of multi-ring aromatics, generally fused multi-ring aromatics or their partially or fully hydrogenated derivatives.
Multi-ring aromatic compounds are characterized as aromatic or hydroaromatic compounds, or both, composed of two or more fused rings. Such compounds include naphthalene and its derivatives (2-rings), anthracene and its derivatives (linear 3-rings), phenanthrene and its derivatives (angular 3-rings), benzanthracenes and benzphenanthrenes (4-rings), indenes, fluorenes, pyrenes, acenaphthenes, and other hydrocarbon compounds of these general types. Hydroaromatics are compounds of these types in which one or more of the rings of the parent multi-ring aromatic compounds have been reduced, or partially reduced with hydrogen. Compounds of these classes which contain three or more fused rings are generally termed polynuclear aromatic hydrocarbons, or PNA's. These types of compounds appear particularly susceptible to color and oxidation reactions which are initiated in the presence of oxygen, particularly on exposure to light; especially, ultraviolet light. Since it is not practical, in many instances, to keep consumer products, e.g., lubes and motor fuels, free from air and light exposure, a simple means to improve oxidative stability would be very useful for improving the ultimate quality of many products derived from petroleum or synthetic petroleum sources, e.g., coal, shale, and tar sands. Thus, there is a profound need for a simple and economical process for improving the color and oxidative stability of streams rich in fused mutli-ring aromatic and hydroaromatic hydrocarbons, particularly lubes basestocks, or stocks useful for lubes and fuels applications.
III. Objects
It is, accordingly, the primary purpose of this invention to fill this need.
In particular, it is an object to provide a process whereby multi-ring aromatic and partialy or fully hydrogenated multi-ring aromatic hydrocarbons, including PNA's and hydro-PNA's, which cause color and oxidative instability in hydrocarbon mixtures can be converted to more stable aromatic molecules.
A further, and more specific object is to provide a process which improves the color and oxidative stability of hydrocarbon products which are normally too color and oxidatively instable to be used in product formulations, especially lubes basestocks formulations.
IV. The Invention
These objects and others are achieved in accordance with the present invention embodying a process wherein a feed comprising an admixture of liquid hydrocarbon compounds, inclusive of a fused multi-ring aromatic hydrocarbon or hydrogenated fused multi-ring aromatic hydrocarbon, or both, containing up to about 10 parts per million parts (ppm) organic sulfur, preferably from about 1 to about 5 parts per million parts, based on the total weight of the feed (wppm) and up to about 10 wppm organic nitrogen, preferably from about 1 to about 5 wppm, based on the weight of total feed, is contacted in the presence of hydrogen over a catalyst which contains elemental iron and one or more of an alkali or alkaline-earth metal [i.e., a Group IA or IIA metal (Periodic Table of the Elements, E. H. Sargent & Co., Copyright 1964 Dyna-Slide Co.)] or compound thereof, and preferably additionally a Group IIIA metal, or metal compound, particularly aluminum, or compound thereof, at temperature sufficient to produce a better color, more oxidatively stable product, as contrasted with said feed. A feed particularly susceptible to such treatment to provide a more color and oxidatively stable product is one containing up to about 5 wppm, organic sulfur, and up to about 5 wppm organic nitrogen, preferably up to about 1 wppm organic sulfur, and up to about 1 wppm organic nitrogen, which feed boils above about 430.degree. F. (224.degree. C.). Preferably, the feed boils within a range of from about 430.degree. F. (224.degree. C.) to about 1000.degree. F. (538.degree. C.), more preferably within a range of from about 650.degree. F. (343.degree. C.) to about 1000.degree. F. (438.degree. C.). Such feed is contacted over the iron catalyst at temperature ranging from about 225.degree. C. (437.degree. F.) to about 430.degree. C. (806.degree. F.), more preferably from about 250.degree. C. (482.degree. F.) to about 350.degree. C., (662.degree. F.) and at hydrogen partial pressures ranging from about 0 psig to about 1000 psig, preferably from about 100 psig to about 600 psig, sufficient to stabilize the product of said reaction against oxygen and light, or color degradation.
In accordance with this invention many hydrocarbon feedstocks which are normally too color and oxidatively instable to be used directly as products, or in product formulations, can be satisfactorily stabilized against oxygen and light to prevent or retard color degradation and/or sediment formation. These include, in particular, (i) jet fuels and diesel fuels, which in tankage are unstable and form sediment upon standing; (ii) hydrocarbon mixtures which fail to meet customer acceptance due to discoloration or sedimentation, or which possess a propensity to become discolored or to throw sediments, or both, on standing, e.g., solvents or lubricating oils; and especially (iii) hydrocracked stocks for lubes applications, which can be stabilized sufficiently to pass certain required stability requirements or specifications.
In particularly preferred operations, organic sulfur and organic nitrogen containing aromatic or hydroaromatic feeds can be hydrotreated to reduce the sulfur and nitrogen concentrations, respectively to very low levels, suitably below about 5 wppm, respectively, and preferably below about 1 wppm, respectively, and the relatively high boiling products therefrom, as feeds, can be treated, in the presence of hydrogen, over the iron catalyst to provide lubes basestocks which possess superior color and oxidative stability. Hydrocracked aromatic or hydroaromatic feeds can be similarly treated in the presence of hydrogen over the iron catalyst to produce lubes basestocks which possess superior color and oxidative stability.
The alkali or alkaline-earth metal promoted iron catalyst required for the practice of this invention can be supported or unsupported, but in either instance the catalytic surface is one which is constituted essentially of metallic, or elemental iron (Fe.degree.) crystallites about which the alkali or alkaline-earth metals are dispersed, generally as a monolayer of an alkaline oxide or alkaline-earth metal oxide. The catalyst is unsulfided, and can function in the presence of sulfur only when a sufficient portion of the catalytic surface of the catalyst is substantially metallic, or elemental iron (Fe.degree.). The formation of sufficiently high concentrations of sulfur at the catalyst surface tends to produce catalyst deactivation via the formation of iron sulfide upon the catalyst surface as a consequence of which the use of feeds which contain high concentrations of sulfur or sulfur compounds should be avoided. High concentration of feed sulfur will soon deactivate the catalyst by converting a major portion of the metallic, or elemental iron surface of the catalyst to iron sulfide.
Whereas applicants do not wish to be bound by any specific theory of mechanism, it is nonetheless believed that the stabilization of products treated in accordance with this invention can be explained. Thus, it is believed that the conventional hydrogenation of fused multi-ring aromatics produces hydrogenated intermediates wherein some of the carbon-carbon and carbon-hydrogen bonds of the parent compounds become more susceptible to reaction with oxygen, often promoted by the exposure to light, than the parent, fully aromatic compounds. Certain phenomena occur as a result of the association between these active sites and oxygen. First, aldehydes, acids, ketones, and the like are formed, these often being highly colored materials. Moreover, as a result of the oxygenation reactions there can be created new multiple order bonds between carbon atoms and carbon/oxygen atoms which increases the number of conjugated bonds in the molecule. The increased conjugation between the carbon atoms increases the formation of color; and where both oxygen and conjugated bonds are present in the same molecule there is a synergistic effect which further intensifies color formation. Furthermore, the oxygen atoms provide reaction sites, e.g., sites for hydroxyl, carbonyl, or carboxyl appendages to the molecule, which react with each other or with other compounds, this leading to the formation of larger molecules which appear as solids dispersed in a liquid phase, or as precipitates.
The process of this invention will be better understood by reference to the following specific, and preferred embodiments and to the drawings which are simplified flow diagrams depicting the various steps of a given process. Valves, pumps, compressors, separators, reboilers, and the like have been omitted from the drawing for clarity.